Interpretive Summary: Fluctuations in precipitation from year to year cause changes in surface water runoff from a watershed. These changes in runoff can either benefit or adversely impact water supplies for agricultural, domestic, industrial and ecosystem uses. A study was conducted to determine how changes in precipitation amounts affect the hydrologic response of a watershed in Southwestern Oklahoma. A computer simulation model referred to as the Soi and Water Assessment Tool (SWAT) was used to predict changes in evapotranspiration and runoff volumes due to precipitation fluctuations on a 208 square mile area of the Little Washita River Experimental Watershed (LWREW). The model simulated a period of 50 years to establish a baseline from which to compare changes in precipitation. For the baseline condition, the model predicted annual precipitation, evapotranspiration, and runoff of 29.6, 27.2, and 2.5 inches, respectively. Next, precipitation amounts were increased or decreased by 20, 40 and 60 percent and a sensitivity analysis compared evapotranspiration and runoff volumes to their baseline values. Simulated runoff varied from 0 inches for a 60 percent decrease in the baseline precipitation to 12.8 inches for a 60 percent increase in the baseline precipitation. Results from this study suggest that at precipitation levels that are nearly half of the average annual amount, runoff from the LWREW becomes negligible. Such a low annual precipitation volume would result in a severe shortage of water with profound impacts to society, wildlife, and aquatic environments.

Technical Abstract:
The Soil and Water Assessment Tool (SWAT) was used to predict variations in precipitation on a 538 square kilometer area of the Little Washita River Experimental Watershed (LWREW) in Southwestern Oklahoma in order to quantify the impact of fluctuations in precipitation on evapotranspiration and runoff from the watershed. Observed precipitation and streamflow data from the watershed were used to calibrate SWAT, and then the precipitation generator in the model was utilized to simulate variations in precipitation that comprised -60, -40, -20, +20, +40, and +60 % of the baseline precipitation volume for the watershed. Expressed as a percentage of precipitation, simulated runoff volumes varied from 0 % to 27 % for -60 % to +60 % changes, respectively, in the baseline precipitation. Test results from this study suggest that at very low levels of annual precipitation approaching 400 mm, runoff from the LWREW becomes negligible. .